Transmission belts are hitherto widely used as a means of transmitting power from a driving side to a driven side, and in these days, synchronous belts, V-belts, V-ribbed belts and the like for automobiles or general industries are required to have a high load transmission capability.
In addition, a system or device has been highly integrated and hence reduced in size to meet the recent demand of saving the space, and specifically pulleys around which a transmission belt is wound are reduced in diameter, which in turn requires the transmission belt to be tolerable against large stress and strain.
Furthermore, the reduction in size of the system or device causes heating parts to be tightly packed in the system or device, which in turn causes the transmission belt to be heated to a temperature higher than ever. Thus, there is a demand for a transmission belt that is more compatible for high temperature use.
Meanwhile, for transmission belts, those having a high tensile member called as core wires embedded therein are widely used to impart tensile strength in a power transmission direction (belt longitudinal direction).
For example, in a V-ribbed belt or the like, these core wires are disposed outside of a compression rubber layer which enables frictional power transmission relative to a pulley and inside of a back-side rubber layer which acts as an outermost layer of the transmission belt.
The core wires are, for example, adhered to a rubber layer called as an adhesive rubber layer formed to have an elastic modulus generally lower than that of the compression rubber layer, and are embedded therein.
In order to satisfy high load transmission capability, strength tolerable against large stress and strain, and heat resistance compatible for high temperature use required for transmission belts, studies are recently under way to use materials having improved heat resistance, cold resistance, frictional resistance, resistance to bending deteriorates and the like for constitutional elements, such as a compression rubber layer, an adhesive rubber layer, core wires, a back-side rubber layer, and sometimes ethylene-α-olefin elastomer is sometimes used for a rubber layer of a transmission belt since it is excellent in heat resistance and cold resistance, and is obtainable at low cost and is compatible for halogen free.
For example, ethylene-propylene-diene-terpolymer (hereinafter referred also as “EPDM”) or the like is used by being crosslinked by an organic peroxide.
For core wires, polyester fibers, aramid fibers, polyamide fibers or the like which are excellent in heat resistance and mechanical strength are generally used.
In order to further provide satisfied characteristics required for a transmission belt, such as high load transmission capability, mechanical strength compatible for large stress and strain, and heat resistance compatible for high temperature use, it is critical not only to use materials having improved heat resistance, cold resistance, frictional resistance, resistance to bending deteriorates for those constitutional elements, but also to improve durability of an interface between the respective constitutional elements.
Among them, it is especially critical to improve the durability of an interface between a rubber layer, such as an adhesive rubber layer to be adhered to core wires, and core wires.
The aforesaid ethylene-α-olefin elastomer, such as EPDM, usually has a lesser polarity and a lower adhesiveness than other rubbers.
Therefore, cracks may take place in an adhesive interface between a compression rubber layer and an adhesive rubber layer, or between core wires and these rubber layers in a transmission belt having a rubber layer made of ethylene-α-olefin elastomer.
An adhesive rubber layer is generally formed by using a soft rubber having such a high fluidity as to be able to be fluidized when heat pressing in molding a transmission belt and hence be securely and tightly adhered to core wires.
On the other hand, a compression rubber layer is generally formed by mixing short fibers in a rubber to have an elastic modulus much higher than that of the adhesive rubber layer from the viewpoint of preventing deformation and improving power transmission capability, of a transmission belt.
Therefore, cracks or the like are likely to take place in an adhesive interface between a compression rubber layer and an adhesive rubber layer, or between core wires and these rubber layers. In order to suppress these cracks or the like, Patent Documents 1, 2 propose that short fibers or carbon black is mixed in a rubber to have an adhesive rubber layer having a higher modulus than ever.
Patent Document 3 proposes that core wires are disposed in an interface between the compression rubber layer and the adhesive rubber layer, and the core wires are adhered to both the rubber layers, namely the compression rubber layer and the adhesive rubber layer, and the resulting product is used for a transmission belt.
Patent Document 4 proposes that an adhesive rubber layer is formed by using a rubber exhibiting a high modulus at a high temperature.
However, when core wires are adhered to a rubber layer having a high modulus in the manner as proposed in those Patent Documents, the rubber layer having a high modulus to be adhered to core wires itself may cause cracks therein.
Also, when the adhesive rubber layer is formed with a high modulus in the manner as proposed in the Patent Documents 1, 2, a problem of dynamic heat build-up of an adhesive rubber layer having such a high modulus may become significant in an actual use in which it is wound around, for example, pulley having small diameter.
Furthermore, when an adhesive rubber layer is formed with a high modulus in the manner as proposed in the Patent Documents, 1, 2, a transmission belt has a high flexural rigidity and hence causes a large hysteresis loss due to the repeated bending. Therefore, a large energy loss is caused in operation and the transmission efficiency may be lowered.
Patent Document 5 proposes that a chlorosulfonated polyethylene or hydrogenated nitrile rubber is used between a rubber layer, for which EPDM is used, and core wires. However, the adhesiveness is not satisfactorily improved in actual and therefore its proposal is hardly practicable.
Specifically, a transmission belt in which a rubber layer is formed by using ethylene-α-olefin elastomer as a base rubber, and this rubber layer and core wires are adhered to each other, has a problem that the durability of the transmission belt is improved while suppressing lowering of a transmission efficiency.
Patent Document 1: Japanese Examined Utility Model Application Publication No. Hei-1-10513
Patent Document 2: Japanese Unexamined Patent Application Publication No. Hei-10-103413
Patent Document 3: Japanese Unexamined Patent Application Publication No. Sho-57-204351
Patent Document 4: Published Japanese Translation of PCT International Application (Tokuhyo) No. 2004-507679
Patent Document 5: Japanese Unexamined Patent Application Publication No. Hei-10-103417